Membrane distillation membranes and modules

As stated previously, the membranes used in MD must be porous and hydrophobic. It can be a single hydrophobic layer (i.e., conventional and most used membrane), a composite porous bilayer hydrophobic/hydrophihc membrane or a composite trilayer hydrophilic/ hydrophobic/hydrophilic or hydrophobic/hydrophilic/hydrophobic porous membranes. Both supported and unsupported membranes can be used in this process. The pore size of the membranes frequently used in MD lies between 10 nm and 1 p,m. 

TABLE 12.1 Commercial Membranes Commonly Used in MD (Membrane Thickness, S Mean Pore Size, Porosity, e Liquid Entry Pressure of Water, LEP ,). 

Membrane Type Membrane Trade Name Manufacturer Material 8 (p-m) dp (pm) s (%) LEP (kPa) 

Flat-sheet membranes TF200 Gelman PTFE/PP 178 0.20 80 282 

Compared to other membrane separation processes such as pervaporation, reverse osmosis, and gas separation, only a few authors have considered the possibility of designing and manufacturing new membranes for MD processes (Khayet et al., 2006a). Moreover, looking into the extended published articles in the MD field, only a few authors have considered the possibility of designing and manufacturing membranes for MD processes. The MD membranes have to meet several requirements simultaneously  

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Figure 13.15 Flow scheme and performance data for a membrane distillation process for the production of water from salt solutions [31]. Feed salt solution is heated to 100 °C and passed counter-current to cool distillate that enters at 42 °C. The distillate product is almost salt-free as shown by its low conductivity. The distillate flux is almost constant up to salt concentrations as high as 20 % NaCI. Reprinted from J. Membr. Sci. 39, K. Schneider, W. Holz, R. Wollbeck and S. Ripperger, Membranes and Modules for Transmembrane Distillation, p. 25. Copyright 1988, with permission from Elsevier...

The catalytic esterification of ethanol and acetic acid to ethyl acetate and water has been taken as a representative example to emphasize the potential advantages of the application of membrane technology compared with conventional distillation [48], see Fig. 13.6. From the McCabe-Thiele diagram for the separation of ethanol-water mixtures it follows that pervaporation can reach high water selectivities at the azeotropic point in contrast to the distillation process. Considering the economic evaluation of membrane-assisted esterifications compared with the conventional distillation technique, a decrease of 75% in energy input and 50% lower investment and operation costs can be calculated. The characteristics of the membrane and the module design mainly determine the investment costs of membrane processes, whereas the operational costs are influenced by the hfetime of the membranes. 

A hyperfiltration process developed by Mobil Oil, now ExxonMobil, for this separation is illustrated in Figure 5.28(b). Polyimide membranes formed into spiral-wound modules are used to separate up to 50 % of the solvent from the dewaxed oil. The membranes have a flux of 10-20 gal/ft2 day at a pressure of 450-650 psi. The solvent filtrate bypasses the distillation step and is recycled directly to the incoming oil feed. The net result is a significant reduction in the refrigeration load required to cool the oil and in the size and energy consumption of the solvent recovery vacuum distillation section. 

MD holds great promise as a unit operation for water desalination, by itself and in conjunction with other processes such as RO and traditional distillation. Advances in membrane chemistry and module design are expected to close the gap between these processes in the near future. 

For the study of the process, a set of partial differential model equations for a flat sheet pervaporation membrane with an integrated heat exchanger (see fig.2) has been developed. The temperature dependence of the permeability coefficient is defined like an Arrhenius function [S. Sommer, 2003] and our new developed model of the pervaporation process is based on the model proposed by [Wijmans and Baker, 1993] (see equation 1). With this model the effect of the heat integration can be studied under different operating conditions and module geometry and material using a turbulent flow in the feed. The model has been developed in gPROMS and coupled with the model of the distillation column described by [J.-U Repke, 2006], for the study of the whole hybrid system pervaporation distillation. 

Winter D., Koschikowski X, Duever D. (2011), Spiral wound modules for membrane distillation modeUing,vahdation and module optimization, Proc. Int. Workshop on Membrane Distillation and Related Technologies, Ravello (So, Italy), Oct. 9-12, pp. 58-59. 

A schematic diagram of the PMR utilizing DCMD is shown in Fig. 21,16. The hybrid system was applied for removal of different dyes from water. The experimental setup was a typical installation for DCMD. The only modification was the incorporation of a UV-A lamp above the feed tank. Thus, the feed tank fulfilled also a role as a photoreactor, in which the photocata-lytic degradation took place. The process was conducted in batch mode. The suspension of a photocatalyst in the treated water was pumped from the feed tank (volume of 3 dm ) using a peristaltic pump through the heater to the capillary PP membrane module. At the same time distillate was pumped through the cooler to the membrane module. The warm feed flowed inside the capillaries, whereas the cold distillate flowed outside the capillaries. Water vapor and volatile compounds present in the warm feed were transferred through the pores of the MD membrane and then condensed/... 

Membranes can also be used to purify a mixmre and attain composition beyond the azeotropic composition. The pervaporation process features a liquid feed, a liquid retentate, and a vapor permeate. While gas-phase membrane processes are essentially isothermal, the phase change in the pervaporation process produces a temperature decrease as the retentate flows through the unit. Since flux rates decrease with decreasing temperature, the conventional pervaporation unit consists of several membrane modules in series with interstage heating. The vapor permeate must be condensed for recovery and recycle, and refrigeration is usually required. Hybrid systems of distillation columns and pervaporation units are frequently used in situations where distillation alone is impossible or very expensive. An important application is the removal of water from the ethanol-water azeotrope. Chapter 14 will discuss the details of design and control of such processes. 

Eal, R, Manna, A.K. Linnanen, L. (2013) Arsenic removal by solar-driven membrane distillation modeling and experimental investigation with a new flash vaporization module. Water Environment Research, 85 (1), 63-76. 

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